1. Field of the Invention
This invention relates to a process for regenerating a chemical copper plating solution by selectively removing chemical plating reaction-inhibiting components from used chemical copper plating solution.
2. Brief Description of the Prior Art
A chemical copper plating solution contains copper ions, a reducing agent for copper ions, a chelating agent for copper ions, and an alkali metal hydroxide as essential components.
In continued use of a chemical copper plating solution having such a composition as above, components having an inhibiting effect upon the chemical plating reaction will accumulate in the plating solution. That is, the copper ions consumed in the chemical plating reaction is supplemented to maintain its optimum concentration. In that case, copper ions are supplemented in the form of an aqueous solution of copper compound. Thus, counter anions to the copper ions will be accumulated in the plating solution by repeated supplementation of copper ions. For the supplementation, copper sulfate is used as the copper compound on an economical ground, and thus sulfate ions are usually accumulated in the plating solution.
As the reducing agent for copper ions, formaldehyde is mainly used also on an economical ground. Formaldehyde can reduce copper ions to metallic copper at the plating, while the formaldehyde itself is oxidized to formic acid thereby, and formate ions are produced. Thus, the formaldehyde consumed in the chemical plating is supplemented to maintain its optimum concentration. Thus, formate ions will be accumulated in the plating solution by repeated supplementation of formaldehyde.
A chemical copper plating solution is generally alkaline and thus absorbs carbon dioxide from air, and accumulates carbonate ions in the plating solution.
Furthermore, an aqueous solution of alkali metal hydroxide is supplied to a chemical copper plating solution to adjust a pH that has changed during the chemical plating reaction. As the alkali metal hydroxide, sodium hydroxide is mainly used on an economical ground.
Anions such as sulfate ions, formate ions, carbonate ions, etc., when accumulated in a chemical copper plating solution, as described above, will capture divalent copper ions, resulting in deterioration of the stability of the plating solution, decomposition of the plating solution, and lowering of mechanical properties of plating film.
Heretofore, when the chemical copper plating solution contains such accumulated anions as above, SO.sub.4.sup.-2 in the plating solution is removed therefrom as Na.sub.2 SO.sub.4 by precipitation, and the resulting plating solution has been reutilized (Japanese Laid-open Patent Specification No. 119428/77). However, these procedures are not preferable on account of very high plating chemical costs and plating solution disposal cost, and also a plating film with good quality has not been obtained.